1. Field of the Invention
The present invention relates to an active matrix display device, and particularly to a technology which is effective for applying to a substrate having an organic insulating film over which a charge holding capacitor is formed.
2. Description of the Related Art
A liquid crystal panel of an in-plane switching (IPS) type (also referred to as lateral electric field type or fringe field switching (EFS) type) displays images by generating an electric field parallel to a substrate between a pixel electrode and a counter electrode at least in a part, and by driving liquid crystal with the electric field, so as to modulate light passing through the liquid crystal sandwiched between a thin film transistor substrate and a counter substrate.
FIGS. 3A to 3C illustrate an IPS type liquid crystal display device described in JP 2007-328210 A. In this liquid crystal display device, an insulating film 110 and an organic insulating film 111 are formed in the stated order from the bottom so as to cover a pad electrode 108 connected to a polysilicon 103 that is an active layer of a thin film transistor 117. On the organic insulating film 111, there are formed a common electrode 113, an inorganic insulating film 114, and a pixel electrode 116 in the stated order from the bottom so as to form a holding capacitor. Further, there are formed a second contact hole 112 opening in the organic insulating film 111 and a third contact hole 115 that is formed by dry etching of the insulating film 110 and the inorganic insulating film 114 as a batch process.
Here, using SiN as the inorganic insulating film 114, it is possible to form the holding capacitor having a large capacitance.
If a numerical aperture of a pixel is increased, there may be a problem of a short circuit between the common electrode and the pixel electrode. For instance, JP 2008-64954 A discloses a technology for preventing a short circuit between the pixel electrode and the common electrode at a contact part of a common potential line in a FFS type liquid crystal display device.
JP 2008-64954 A describes a structure of a contact hole having a taper angle of 60 degrees or larger, which opens in an interlayer insulating film, for connecting a common potential line with a pad electrode. With this structure, it is possible to prevent diffused reflection when exposing a smoothing layer which is disposed over the contact hole and which is made of a photosensitive resin, so that a recess of the smoothing layer and a short circuit between the common electrode and the pixel electrode caused by the recess may be prevented.
In the conventional technology illustrated in FIGS. 3A to 3C, the insulating film 110 and the inorganic insulating film 114 are made of SiN, and the inorganic insulating film 114 is formed on the organic insulating film 111. Therefore, they may not be formed at high temperature, and the inorganic insulating film 114 is formed at a temperature lower than film forming temperature of the insulating film 110. As a result, the inorganic insulating film 114 is not as dense as the insulating film 110. Therefore, when the inorganic insulating film 114 and the insulating film 110 are processed by dry etching, an etching rate becomes higher in the inorganic insulating film 114 than in the insulating film 110. In this case, as described in JP 2007-328210 A, an edge of the inorganic insulating film 114 may be retracted from an edge of the insulating film 110.
In particular, if a pixel size is reduced so as to increase the resolution of the liquid crystal display device by using the above-mentioned conventional technology, a distance denoted by d in FIG. 3B, which is between the edge of the second contact hole 112 on the common electrode 113 side and the edge of the third contact hole 115 on the common electrode 113 side is decreased, which causes the following problem.
That is, because the etching rate in the inorganic insulating film 114 is faster than the etching rate in the insulating film 110, if the distance between the edge of the second contact hole 112 and the edge of the third contact hole 115 is small, the edge of the inorganic insulating film 114 may be retracted to the organic insulating film 111.
This situation is described with reference to FIG. 3C. At the left side of the third contact hole 115, the inorganic insulating film 114 is retracted to the organic insulating film 111, and hence an exposed part 118 of the organic insulating film is formed.
When the liquid crystal display device having the exposed part of the organic insulating film is operated in an environment of high humidity, moisture in the atmosphere intrudes in liquid crystal 20 illustrated in FIG. 4. In addition, if the moisture intrudes from the exposed part 118 of the organic insulating film into the organic insulating film 111, a resistance of the organic insulating film 111 is decreased so that current leak occurs between the pixel electrode 116 and the common electrode 113. As a result, it becomes difficult to hold a voltage that is applied to the liquid crystal 20, and there occurs a problem that display image quality of the liquid crystal display device is deteriorated. In addition, even in an environment without high humidity, if it is operated for a long term, moisture in the atmosphere may intrude into the liquid crystal 20 so that the same problem as described above may occur.
The technology described in JP 2008-64954 A may prevent a short circuit between the pixel electrode and the common electrode. However, because the distance between the edge of the contact hole opening in the insulating film formed on the organic insulating film and the edge of the contact hole opening in the organic insulating film is small, an exposed part of the organic insulating film is formed. Therefore, the same problem as described above may occur in an operation in an environment of high humidity or in an operation for a long term.
A problem to be solved by the present invention is to prevent current leak of a charge holding capacitor constituting a pixel of a display device so as to prevent deterioration of display image quality in the display device.